1. Technical Field
The present invention relates to an optical article suitable for use as a plastic lens for spectacles, cameras, and the like, and also to a method for producing the same.
2. Related Art
As compared with glass lenses, plastic lenses are lightweight, have excellent moldability, workability, dye affinity, and the like, and are resistant to breakage and highly safe. On this account, plastic lenses have been rapidly spread in the field of spectacle lenses, and now account for a large share. In recent years, in order to meet the increasing demands for thinner and lighter lenses, high-refractive-index materials, such as thiourethane-based resins and episulfide-based resins, have been developed.
JP-A-2004-002712 and JP-A-2005-281527 propose a method for producing an episulfide-based resin with a high refractive index, the method including polymerizing an epithio-group-containing compound (episulfide compound) in the presence of sulfur. Such an episulfide-based resin can readily develop a high refractive index of 1.7 or more, and thus is effective in providing spectacle lenses with reduced thickness.
A known example of an optical article, such as a spectacle lens, is one having a primer layer that is formed on the surface of a plastic substrate and a hard coating layer that is formed on the surface of the primer layer; the primer layer mainly serves to provide adhesion and shock resistance, and the hard coating layer mainly serves to provide scratch resistance and abrasion resistance. With respect to such an optical article, when the plastic substrate has an increased refractive index, in order to prevent the formation of interference fringes, the primer layer and the hard coating layer also have to have refractive indices close to that of the plastic substrate.
According to a common practice, for example, any of various metal oxides is included as a filler in a hard coating layer to increase the refractive index thereof. As such metal oxides, titanium oxide, zirconium oxide, antimony oxide, tin oxide, and the like are usable alone or in the form of composite particulates. Metal oxides generally have poor light resistance and are susceptible to staining. Therefore, considering transparency in the visible region, stability, and the like, titanium oxide is chiefly used.
However, titanium oxide is photocatalytic under ultraviolet radiation. When used as a filler in a hard coating layer, titanium oxide will decompose the binder component formed of the surrounding organic resin, causing separation of the hard coating layer. For this reason, as described in JP-A-2007-102096, in a hard coating layer, titanium dioxide is often employed not in the anatase form, which is likely to exhibit photocatalytic activity, but in the rutile form, which has less photocatalytic activity.
When the refractive index varies greatly between a plastic substrate and a primer layer and/or between a primer layer and a hard coating layer, the difference in refractive index leads to the formation of interference fringes. In recent years, with an advent of high-refractive-index plastic substrates having an refractive index of more than 1.7, for the purpose of suppressing interference fringes, attempts have been made to provide, not only a hard coating layer, but also a primer layer with a high refractive index.
In order to increase the refractive indices of a hard coating layer, a primer layer, and like layers (functional layers) laminated on a lens substrate, mainly, the proportion of metal oxide particulates (metal oxide sol) in a composition forming each layer has to be increased.
Meanwhile, high-refractive-index lens materials are relatively vulnerable, so the hard coating layer and the primer layer, especially the primer layer, are desired to have sufficient shock resistance (shock absorption). The shock absorption of a primer layer is imparted by a resin component in a composition for forming the primer layer. However, when the proportion of metal oxide particulates is, increased in an attempt to achieve a high refractive index, the shock absorption may be insufficiently exhibited.